The phase transition in ${\mathrm{BaSi}}_2$ , an orthorhombic structure at ambient conditions, was investigated by in situ x-ray diffraction along the paths of compression up to 5.2 GPa at room temperature and subsequent heating up to 873 K at 5.2 GPa. No phase transition occurred during the compression at room temperature. When heated at 5.2 GPa, orthorhombic ${\mathrm{BaSi}}_2$ transformed into trigonal ${\mathrm{BaSi}}_2$ at 673 K, whose structure is the same as that of the sample quenched from the same conditions. These results ensure that trigonal ${\mathrm{BaSi}}_2$ forms directly from orthorhombic ${\mathrm{BaSi}}_2$ at high pressures and high temperatures and maintains its structure when the pressure-temperature conditions are returned to ambient conditions. The reason why trigonal ${\mathrm{BaSi}}_2$ is quenched, despite it being a metal, is discussed.

DOI:https://doi.org/10.1103/PhysRevB.55.132